LOS ANGELES (Reuters) - "Obsessed," a new "Fatal Attraction"-style thriller starring R&B singer Beyonce Knowles, easily took the crown at the weekend box office in North America, selling an estimated $28.5 million worth of tickets, its distributor said on Sunday.

The movie cost about $20 million to make, and industry observers were forecasting an opening in the mid-teen millions, said Screen Gems, the low-budget division of Sony Corp's Columbia Pictures.

Knowles, the former Destiny's Child frontwoman who recently played blueswoman Etta James in "Cadillac Records," stars in "Obsessed" as a woman who fights to save her marriage when a pretty office temp (Ali Larter) falls hard for her husband (Idris Elba).

Last weekend's champion, the "Big"-style body-swap comedy "17 Again," slipped to No. 2 with $11.7 million. The movie, starring former "High School Musical" hunk Zac Efron, has earned $40 million after 10 days. It was released by Warner Bros. Pictures, a unit of Time Warner Inc.

The top-10 boasted three other new releases. "Fighting," starring Channing Tatum as a street-brawler, opened at No. 3 with $11.4 million; "The Soloist," a drama starring Jamie Foxx and Robert Downey, Jr., at No. 4 with $9.7 million; and the documentary "Earth" at No. 5 with $8.6 million, the biggest opening for a nature film.

"Fighting" was produced by Rogue Pictures, a unit of closely held Relativity Media, and distributed by Universal Pictures, a unit of General Electric Co's NBC Universal. "The Soloist" was produced by closely held DreamWorks SKG, and distributed by Viacom Inc's Paramount Pictures. "Earth" was produced by Walt Disney Co's nascent Disneynature Films.

(Editing by Philip Barbara)

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